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Source: https://www.builder.io/blog/visual-guide-to-nodejs-event-loop

There are 6 different queue is the node environment

1. Timer Queue -> holds callback with setTimeout and setInterval functions 2. IO Queue -> holds methods asscoiated with Network and IO Operations 3. Check Queue -> Specific to NodeJs, for functions with setImmeditate 4. Close Queue -> Close event of an async task

The above queues are all part of libuv

5. Microtask Queue contains two other queues: a. Promise Queue b. NextTick Queue

github.com/persona-mp3

Order of Execution

From the article, the flow of execution and prirotity of these queues is somewhat in this manner

  1. V8 Executes normal code
  2. When Stack is empty it checks the MicroTaskQueue specifically the nextTick and then promise queue and feeds the callback functions to the V8 Stack
  3. Then the Timer queue is checked, fed to the v8 stack and after, the MicroQueues are checked again

And So on

1. main_stack -> micro_queue
            executes [next_tick and promise_queue]
2. micro_queue -> timer_queue
3. timer_queue -> micro_queue
4. micro_queue -> io_queue
5. io_queue -> micro_queue
6. micro_queue -> check_queue
7. check_queue -> micro_queue
8. micro_queue -> close_queue
9. close_queue -> micro_queue